There are known various forms of disk driving devices configured to record and/or reproduce information by rotating a magnetic recording medium in the form of a disk (hereinafter called a magnetic disk, for example.) Among others, a disk driving device also called "hard disk device" is particularly used in a small-scaled, large-capacity system. Such a hard disk device is configured to rotate at a high revolution a magnetic disks each made of a disk-shaped hard material having magnetic recording layers on surfaces thereof, and magnetic heads are opposed to the surface of the magnetic disk to record or reproduce signals thereon.
FIG. 9 shows one form of the disk driving device of this type. The disk driving device generally comprises magnetic disks 1 on which information is recorded, magnetic heads 2 which record or reproduce information on or from the magnetic disk 1, a direct drive motor (not shown. Hereinafter called "DD motor") which drives the magnetic disks 1, a head driving mechanism 4 which moves the magnetic heads 2 to predetermined tracks on the magnetic disks 1, a support board 5 which supports a housing sealingly accepting therein the magnetic disks 1, the magnetic heads 2 and other members, a printed board 6 on which a motor driving circuit, control circuit, etc. are printed, and a frame (not shown) which holds the printed board 6 on the support board 5.
The illustrated magnetic disk device includes two magnetic disks 1. Each magnetic disk 1 has two recording surfaces on opposite planar surfaces thereof. Therefore, the illustrated disk mechanism includes four magnetic heads 2 associated with respective recording surfaces of the magnetic disks 1. The magnetic heads are mounted on a swing arm 8 of the head driving mechanism 4 by cantilever springs. The head driving mechanism 4 consists of the swing arm 8, a steel belt 9 mounted on a part of the swing arm 8, a pulley 10 on which an intermediate portion of the steel belt engages, and a stepping motor 11 which has a drive shaft 12 supporting the pulley 10 combined with the steel belt 9, so that when the stepping motor 11 is driven, the swing arm 8 swings about a pivot pin 8a thereof.
The magnetic disks 1, magnetic heads 2, swing arm 8, steel belt 9 and pulley 10 are accepted in the casing which consists of the support board 5 and a top cover (ot shown). To establish an airtight sealing of the housing, gaskets are used at the contact between the support board 5 and the top cover and at the mounting portion of the stepping motor 11. Further, magnetic fluid is applied around the shaft of the DD motor for the same purpose. The swing arm 8 is provided with a shutter 17 extending outwardly away from the magnetic heads 2. Nearer to an airtight chamber of the support board 5 is provided a photo interrupter 18 serving as an outside sensor. The photo interrupter 18 defines an insertion path 18a which receives the shutter 17 loosely. In the prior art arrangement, when a magnetic head 2 reaches a zero track position at an outermost circumference, the shutter 17 blocks the light path provided in the insertion path 18a of the photo interrupter 18.
In the arrangement using the stepping motor 11 to transport the magnetic heads 2, head positioning is difficult when the track density of the disks is increased. More specifically, since different materials in the hard disk apparatus have different expansion coefficients, there occurs a problem called "thermal off-track" in which the position of the magnetic head 2 relative to the tracks varies with temperature. Therefore, in a 5.25 inch-type hard disk apparatus, it is difficult to precisely position the magnetic heads 2 beyond 400TPI unless a servo system is used.
U.S. Pat. No. Re. 32,075 discloses an invention of a servo-control system. The system uses a data-masked servo sector including track center line servo-control data detected by a head to fix the position of the head according to one piece of servo information per one revolution of a magnetic disk. Since this servo-control system invites a decrease in the data recording length by an amount corresponding to the servo information, it is configured to slightly slow down the revolution to adjust the head transport speed. This arrangement, however, may invite an instable movement of the head and may increase the error rate. Additionally, since only one piece of servo information is provided in one cycle, it takes a time to detect the servo information after movement of the head.
U.S. Pat. No. 4,122,503 discloses another control system using a servo system in which the inner-most and outer-most tracks are used as particular servo tracks. This system is called "ID-OD system" in abbreviation of "inner diameter" and "outer diameter". In this system, the disk apparatus is conofigured to first read the outer servo track and effect fine adjustment to place the head at the center of the track. Subsequently, the head is moved toward the inner servo track. In this operation, step pulses of the stepping motor in the head driving mechanism are counted, so that when the head reaches the inner servo track, the head positioning mechanism effects precise positioning to bring the head at the center of the track. While the precise positioning is effected for each servo track, the positioning mechanism is informed of a correction amount necessary for finding the center of the track. Obtaining the correction amount, the positioning mechanism is enabled to calibrate precise positions of respective tracks according to information about the number of step pulses required for movement between the outer and inner tracks and the fine step correction amount required in each servo track.
Further, the magnetic disk apparatus records information by saturation recording.
Saturation recording 2 is such that the current applied to each head for its information writing is larger than a current value which saturates the magnetization of the magnetized layer of the magnetic disk in one direction. The saturation recording features in that new information can be written by "over-writing" which does not require erasure of old information on the disk before writing the new information. This simplifies the head construction and enables an instantaneous changeover between reading and writing operations. Therefore, a single track may be divided into multiple sectors so that reading and writing may be effected per each sector, and this contributes to the maximum use of the recording surfaces without loss.
In order to write or read information on a magnetic disk, it is necessary to make a format in an information recording region on the magnetic disk. The format may be a known format called "floppy-like" format, for example, in which one cycle from first to fourth gap is divided into 32 sectors related to an exterior index signal EIN.
The exterior index signal EIN corresponding to the first gap is settled at a position where a predetermined count number is detected from detection of a first interior index signal IN1 which is supplied from a Hall element or other magnetic detecting means upon detection of the rotational position of a pulse generating magnet 40 attached to a rotor of the DD motor 3. That is, when the said count number is detected after detection of the interior index signal IN1, the exterior index signal EIN is applied to the host computer 26, and this position is regarded as the beginning of the recording track T.
Such a format is usually formed in the recording regions except those having servo information thereon before shipment of the system, and a formatting is effected by an end user when he first uses the magnetic disk driving device having the magnetic disk therein to enable information writing in the data field of the format.
In an ID-OD or other system in which servo information is provided in limited tracks alone, positional control of the head is effected based on the servo information which is written in a relationship with exciting phases of a stepping motor or other device for transporting the magnetic head. Therefore, if the stepping motor mis-steps for any reason, it sometimes occurs that the formatting is written, erasing the servo information. If the formatting is once written on the servo information, the system cannot obtain the servo information thereafter, and cannot position the head accurately.
Further, disk driving devices known heretofore are configured to detect the zero track of a magnetic disk, using a sensor or other mechanical means, or alternatively using a particular signal specifically written in the radial direction for zero track detection to unable rewriting in the region having the particular signal.
However, the use of such a mechanical means invites an increase of the manufacturing cost of the system because of an additional cost not only for the sensor or other part itself but also for a more difficult assembling process caused by more complicated zero track positional adjustment. The use of the particular signal to unable rewriting thereon necessarily requires a slow-down of the disk rotation to adjust the transporting speed, and this causes an instability of the head assembly and an increase of the error rate. This problem also remains in the aforegoing U.S. Pat. No. Re. 32,075.